We shall define here "similar" solutions as those for which the velocity and the temperature profiles at all different values of x can be made congruent if they are plo- tted in coordinates that have been made dimensionless with reference to the suitable scale factors. Obviously, when similar solutions exist, it is quite possible for the System of partial differential equations to reduce to ordinary differential equations, which, evidently, constitute a considerable mathematical simplification of the problem. The quest for similar solutions, therefore, is of paramount importance. Finally, the existent conditions of similar solutions are investigated and the methods to transform the partial differential, equations of boundary layer to an odinary differential equations of similar solutions are proposed.